In recent years, thin film solar cells which use amorphous silicon or chalcogenide-based compounds as materials have come under the spotlight. Such solar cells have the superior properties of being inexpensive in cost of materials and enabling easy production of large-sized solar cell panels. On the other hand, in general, their photovoltaic conversion efficiency is inferior to that of crystal-type solar cell. Much more improvement is desired.
In crystal-type solar cells which use silicon wafers as materials, to further improve their photovoltaic conversion efficiency, it has been proposed and commercially practiced to make the electrode layer a point contact structure. The contact interface between the semiconductor layer and the electrode layer is a part with a high density of dangling bonds and other crystal defects and where the carrier recombination speed is the fastest. Therefore, in the prior art, the light absorption layer (semiconductor layer) and the electrode layer are made to contact each other by point contact to reduce the ratio of surface recombination and improve the photovoltaic conversion efficiency. Specifically, by forming a good quality oxide film or nitride film with a small surface recombination rate which would function as a passivation film at the majority of the interface between the light absorption layer and the electrode layer, point contact would be realized and the ratio of recombination of carriers would be reduced (for example, see PLT 1). Due to this, it is known that the open circuit voltage, which is one of the solar cell characteristics, becomes higher and the photovoltaic conversion efficiency is improved.
However, in current thin film solar cells, the above such point contact structure is not realized. When applying the above art designed for crystal silicon solar cells to, for example, CIS-based thin film solar cells, it is necessary to form an insulation film with few defects between the semiconductor layer and the electrode layer, but the art of forming such an insulation film has not been realized. There is a possibility that it will be realized by future technical innovations, but even in that case, probably the process of production would become complicated and an increase in manufacturing costs would be invited.
Therefore, in a thin film solar cell, if it were possible to realize a point contact structure which is easily formed between the light absorption layer and electrode layer, improvement of the photovoltaic conversion efficiency of the thin film solar cell would be achieved without inviting an increase in the cost of manufacture.
PLT 1: Japanese Patent Publication No. H9-283779A
PLT 2: Japanese Patent Publication No. 2009-246025A